Experimental Evidence for Heavy-Atom Tunneling in the Ring-Opening of Cyclopropylcarbinyl Radical from Intramolecular 12C/13C Kinetic Isotope Effects Page: 12,548
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Published on Web 08/19/2010
Experimental Evidence for Heavy-Atom Tunneling in the Ring-Opening of
Cyclopropylcarbinyl Radical from Intramolecular 12C/13C Kinetic Isotope Effects
Ollie M. Gonzalez-James,t Xue Zhang,t Ayan Datta,t David A. Hrovat,t Weston Thatcher Borden,*t'
and Daniel A. Singleton*t
Department of Chemistry, Texas A&M University, College Station, Texas 77842, and Department of Chemistry and
Center for Advanced Scientific Computing and Modeling, University of North Texas, 1155 Union Circle #305070,
Denton, Texas 76203-5017
Received June 24, 2010; E-mail: singleton@ mail.chem.tamu.edu
Abstract: The intramolecular 13C kinetic isotope effects for the
ring-opening of cyclopropylcarbinyl radical were determined over
a broad temperature range. The observed isotope effects are
unprecedentedly large, ranging from 1.062 at 80 C to 1.163 at
-100 C. Semiclassical calculations employing canonical varia-
tional transition-state theory drastically underpredict the observed
isotope effects, but the predicted isotope effects including tun-
neling by a small-curvature tunneling model match well with
experiment. These results and a curvature in the Arrhenius plot
of the isotope effects support the recently predicted importance
of heavy-atom tunneling in cyclopropylcarbinyl ring-opening.
Tunneling is a fundamental physical phenomenon that affects
the rates of barrier crossings in all chemical reactions. In most
reactions, the effects of tunneling are subtle, and the impact of
tunneling on experimental observations is not readily recognizable.
This is particularly true of reactions involving heavy-atom tunneling,
since the probability of tunneling decreases rapidly with increasing
mass. The theoretical support for heavy-atom tunneling is unques-
tioned, but its experimental manifestations are less obvious.
The most common evidence for heavy-atom tunneling has been
the observation of reactions that occur at cryogenic temperatures,
where there is little thermal energy to allow passage over a reaction
barrier.1-4 Without tunneling, calculations show that passage over
the barrier is much too slow to account for the measured reaction
rates, but the inclusion of tunneling provides calculated rate
constants that are in good agreement with those that have been
Accurate theoretical calculations can be used to predict the
contribution of tunneling to a reaction at any temperature. However,
for reactions that are carried out at noncryogenic temperatures, the
question of what experimental observations, by themselves, reliably
implicate heavy-atom tunneling remains open.
We describe here an experimental study of the intramolecular
13C kinetic isotope effect (KIE) for the ring-opening of the
cyclopropylcarbinyl radical (3)." The results reveal an unprecedent-
edly large 13C KIE and confirm recent predictions of an important
role for tunneling in this reaction. A careful scrutiny of the
temperature dependence of the KIE reveals an experimental
diagnostic of heavy-atom tunneling in the ring-opening of 3 that
does not depend on a comparison between predicted and measured
Datta, Hrovat, and Borden have recently predicted that tunneling
should make the ring-opening of 3 fast at 20 K.6 A narrow reaction
t Texas A&M University.
University of North Texas.
12548 * J. AM. CHEM. SOC. 2010, 132, 12548-12549
barrier is calculated to allow tunneling to occur rapidly, without
any thermal activation, from the molecule's vibrational ground state.
We sought to determine if tunneling in the ring-opening of 3 would
be apparent in experimental observations made at higher and more
routinely accessible temperatures in solution.
Toward that end, we studied the intramolecular 13C KIE7 for
the ring-opening of 3. As probes for tunneling, intramolecular KIEs
have significant advantages over absolute rate measurements;
intramolecular KIEs reflect only the first irreversibly desymme-
trizing step in a mechanism, and they can be readily determined
precisely at a broad range of temperatures. To measure the
intramolecular KIEs in the ring-opening of 3, samples of bromom-
ethylcyclopropane (1) at natural abundance were reduced at
temperatures ranging from -100 to 80 OC under free-radical chain
conditions, employing tributyltin hydride as the reductant and
triethylborane/02 as the initiator. This reduction affords 1-butene
(2) as the major product, arising from the ring-opening of 3, along
with small amounts of methylcyclopropane (5), formed by the
reduction of 3 before ring-opening. Under the conditions employed,
the ring-opening of 3 should be irreversible (see the Supporting
Information for a discussion).
[> CH2Br 3 H3 CH2
Et3B, 02 2
H s CH2
The relative 13C content in the C-3 versus C-4 positions of 2 was
analyzed by NMR.7 In all cases, less 13C was observed at the C-4
position of the 1-butene than at C-3, reflecting a preference for 12C to
undergo the bond-breaking process that places it in C-4 of the ring-
opened radical 4. The intramolecular KIEs that are defined by the ratios
of 13C at C-3 and C-4 are summarized in Table 1.
The KIEs in Table 1 are strikingly large. Even at 80 OC, the 13C
isotope effect is over 6%, and the 13C KIE ultimately increases to
16% at -100 C. Larger 13C magnetic isotope effects have been
observed,8 but among conventional 13C KIEs, those in Table 1 are
the largest known at each temperature.
In order to investigate the role that tunneling plays in the 13C
KIEs in Table 1, the KIEs were computed both with and without
inclusion of tunneling. Canonical variational transition-state theory
(CVT)9 was used to calculate the semiclassical rate constants,
without tunneling, and the effect of multidimensional tunneling on
the reaction dynamics was computed using the small-curvature
10.1021/ja1055593 2010 American Chemical Society
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Gonzalez-James, Ollie M.; Zhang, Xue; Datta, Ayan; Hrovat, David A.; Singleton, Daniel A. & Borden, Weston T. Experimental Evidence for Heavy-Atom Tunneling in the Ring-Opening of Cyclopropylcarbinyl Radical from Intramolecular 12C/13C Kinetic Isotope Effects, article, August 19, 2010; [Washington, D.C.]. (https://digital.library.unt.edu/ark:/67531/metadc71805/m1/1/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.